How to use Chacha as key scheduling algorithm for any other algorithm

[Moh-Gebril]

Hi, I want to use Chacha as key scheduling algorithm for any other algorithm, It will be very helpful if you provide and example for integrating Chacha as key scheduling algorithm for any of the existing CLAASP's implemented algorithms.
Thank you in advance.
Kind Regards,
Gebril

[p-huynh]

Hi @Moh-Gebril,

With CLAASP you cannot directly create a cipher object that mixes the key schedule of one cipher with the round function of another. To do this, you’d need to either:

1. Manually implement a new cipher class, reusing the Chacha and the other cipher’s components, or
2. Use an alternative approach where you let Chacha generate round keys and feed those into the second cipher. I will give more details below.

---

Using Chacha as a round-key generator

If you want to use the Chacha permutation to derive round keys, here’s how:

• Load the master key into the Chacha state.
• Each Chacha round produces an output that you can use as a round key.
• Pass these precomputed round keys into another cipher that has its key schedule removed.

---

Step 1. Run Chacha to generate round outputs

Example with a toy Chacha using 8-bit words (instead of the usual 32-bit):

from claasp.ciphers.permutations.chacha_permutation import ChachaPermutation

N_ROUNDS = 4
secret_key = 0x01_02_03_04_05_06_07_08_0a_0b_0c_0d_0e_0f  # 16-byte key

chacha = ChachaPermutation(
    number_of_rounds=N_ROUNDS,
    rotations=[2, 1, 4, 3],
    word_size=8
)

# Evaluate and collect intermediate outputs
chacha_outputs = chacha.evaluate([secret_key], intermediate_output=True)

chacha_outputs is a tuple with 3 elements:

1. The final cipher output
2. A dict with all round outputs + final output
3. A dict with the inputs/outputs of each internal component

From (2), you can extract all round outputs:

round_keys = chacha_outputs[1]['round_output'] + chacha_outputs[1]['cipher_output']

---

Step 2. Create Speck without a key schedule

from claasp.ciphers.block_ciphers.speck_block_cipher import SpeckBlockCipher

speck = SpeckBlockCipher(
    block_bit_size=32,
    key_bit_size=64,
    number_of_rounds=N_ROUNDS
).remove_key_schedule()

Check inputs:

speck.inputs_size_to_dict()
# {'plaintext': 32, 'key_0_2': 16, 'key_1_2': 16, 'key_2_2': 16, 'key_3_2': 16}

Notice: instead of a single master key, Speck now expects 4 round keys of 16 bits each.

---

Step 3. Truncate Chacha outputs to 16 bits

Since Chacha produces 32-bit values, reduce them to 16 bits:

shortened_round_keys = [x & 0xFFFF for x in round_keys]

---

Step 4. Evaluate Speck with custom round keys

plaintext = 0x74614620
speck.evaluate([plaintext] + shortened_round_keys)

---

This way, you effectively use Chacha to generate round keys and Speck to process them, without having to implement a new cipher class manually.

[Moh-Gebril]

Thank you @p-huynh, I am really appreciated.